This invention relates to the field of ferrite compositions used as browning elements in a microwave oven for browning or crisping food. More particularly, the ferrite compositions are used in a microwave oven dish or laminate to maintain the dish or laminate at a desired temperature for browning or crisping food.
Microwave ovens have been popular for many years because they heat food much faster than conventional ovens and consume less energy. However, one of the previous drawbacks for microwave cooking was the difficulty in obtaining a crust of browning food. Recent developments have made significant improvements in this area. Specifically, at least one microwave oven manufacturer now includes reusable crisping/browning elements consisting of ferrite powders embedded in plastic or rubber (see U.S. Pat. No. 5,268,546). Several manufacturers sell a metallic paper throw-away item to wrap food for crisping/browning (see e.g. U.S. Pat. No. 5,285,040).
A ferrite material currently used in reusable microwave browning dishes known as manganese zinc ferrite includes manganese, zinc, and iron oxide. Ferrite powders used for microwave crisping applications such as manganese zinc ferrite are quite expensive. These ferrite powders use a high percentage of costly raw materials such as manganese and zinc oxide. Further, these ferrite powders must be sintered in atmospheres other than air, such as nitrogen atmosphere, to prevent the manganese from converting to a higher valence during the sintering and cooling process. Special atmosphere furnaces cost 40% to 100% more than air furnaces. Also, maintenance for special atmosphere furnaces costs more than maintenance for air furnaces. Additionally, very tight control of temperature, time, and oxygen percentage is required in the process of sintering manganese zinc ferrite to create a material that will crisp food in a microwave oven. Thus, there is a need for a low-cost ferrite material for use in a microwave oven browning device.